1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a toothed belt having a fabric layer thereon that resists tooth abrasion and chipping, particularly in a high temperature environment in which the belt is heavily tensioned or under a heavy load.
2. Background Art
Toothed power transmission belts are well known in the art. It is known to construct the teeth and a backing layer on these belts from a rubber blend including chloroprene rubber. It is also known to incorporate into these belts load carrying members made from glass fiber cord or aramid fiber cord. The individual fibers in these cords are adhered together using an RFL (resorcinol-fonnalin-latex) solution containing a latex component of vinylpyridine latex from the SBR group. It is also known to place a fabric covering layer on the inside surface of the belt over the belt teeth. One type of fabric has a woven warp and weft, with the latter being a crimped nylon material.
The above type of toothed belt is what is conventionally used for vehicle OHC (overhead cam) drive belts. These belts are typically operated at elevated temperatures and even more so of late by reason of the trend towards use of these belts in small front engine compartments on front wheel drive vehicles. The conventional belts made from chloroprene rubber and operating in this environment are prone to a number of defects. Tooth chipping may be caused by cracks developed in the belt backing and at the tooth flank. Further, glass load carrying members tend to rapidly degrade under repeated flexing of the belt at these high temperatures. Either belt defect may lead to premature belt failure.
One attempted solution to the cracking and chipping problem discussed above, has been to modify the rubber composition defining the belt teeth and backing layer. In Japanese Patent Publication JP-A-62-159827, different rubber compositions are disclosed to alleviate the above problems and thereby prolong the belt life in a vehicle application. It is taught therein to use a rubber blend including chlorosulfonated polyethylene (CSM) and hydrogenated nitrile rubber (HNBR) prepared by hydrogenating a copolymer of acrylonitrilebutadiene.
It is further known to change the characteristics of the load carrying members and fabric coveting layer and to combine these improved load carrying members and covering layer with the modified rubber blends to eliminate cracking and chipping in the toothed belts. The coveting layer reinforces the tooth flank that is subjected to a concentrated stress with the belt under a heavy load and/or highly tensioned. However, if the covering layer abrades, the reinforcing effect of the covering layer is reduced, as a result of which the concentrated stresses at the tooth flanks induce chipping and cracking.
In Japanese Patent Publication JP-A-55-36642, a canvas fabric layer is disclosed which is woven using polyester yarn or aramid fiber for one of the warp or weft and 6-nylon or 6,6-nylon for the other of the warp and weft. It is explained that this canvas covering layer improves heat resistance.
In Japanese Utility Model Publication JP-U-62-183 147, a fabric covering layer is disclosed wherein the weft contains at least woven woolly finished yarn of aramid fiber and urethane elastic yarn.
Another fabric cover layer aimed at improving resistance to abrasion and tooth chipping is disclosed in Japanese Patent Publication JP-A-4-8948. The fabric is described therein as a) a combined twisted yarn of aramid spun yarn and urethane elastic yarn for the weft and b) filament yarn of an aliphatic group fiber or aramid fiber for the warp.
A fabric covering layer using a conventional woolly finished yarn of aramid fiber has an irregular, towel-like surface. The woolly finished yarn produces a random thickness. This results from the inability to produce uniform twists due to the temperature variations during processing and the rigidity of the fiber. The irregular surface of the canvas covering layer results in a non-uniform PLD value over the length of the belt (the PLD value is the distance between the pitch line passing through the center of the load carrying cords and the belt bottom land). A further consequence of this is that belts made from a single mold may vary in length.
Under normal operating conditions, a fabric cover layer which has a combined twisted yarn of aramid spun yarn and urethane elastic yarn as the weft improves abrasion resistance and resistance to cracking and tooth chipping. However, when this type of belt is made in a narrow form or used under heavy loads, or when the variation of load is significant as in the case of a diesel engine drive, the abrasion on the teeth becomes particularly severe. The problem is aggravated with the belt operating in a high temperature environment, i.e. 100.degree.-140.degree. C. As a result, the tooth flanks tend to crack and chip after a relatively short period of operation by reason of the stress concentration thereat.
Another problem with conventional belts using cover fabric is that the cover fabric employs 6-nylon yarns or 6,6-nylon yarns which may melt and adhere to a cooperating pulley as the belt operates for long periods at temperatures in the 100.degree.-140.degree. C. temperature range. This causes the cover layer to wear down prematurely which leads to crack formation in the teeth.